On Mediawikiwiki, I initiated a new post in Flow, and the editing interface defaulted to VE. This was despite my existing user preference "Temporarily disable VisualEditor while it is in beta". Instead, I had to manually select wikitext editing interface, and the "icon" used was not intuitive. (The icon is </>) It was only through persistence that I was able to figure it out.
OS: Windows 7, Browser FF38.0.5 - only Wikitext option appears when using Win7, IE9 (which I believe is standard for that very old browser).